This invention concerns aluminium workpieces, particularly aluminium sheet, mainly thin sheet used for containerstock or as foil. Containers are often subjected to abrasion during transportation prior to filling.
Foil containers are frequently formed, stacked together and then transported to a separate site for filling. The process of transportation results in fretting between adjacent containers. Cleaning the foil stock and then coating the surface with an abrasion resistant coating can prevent fretting, but the economics of foil production require that any coating process be of low cost.
Cleaning of foil stock at high speed is difficult. Annealing alone does not give adequate cleaning, and chemical rinses are often too mild, i.e. they only degrease. Electrolytic processes are probably the only ones that can clean a foil surface in the short process times required.
Currently, foil for food use is sold as a commodity product mainly in the bare state or backed with paper or polymer as a laminate, sometimes with simple lacquer printing. Generally, printed or laminated products are suitable only for cold applications, e.g. sandwich wrap, while bare metal is also frequently employed in food ovens. Few coatings are available that on heating, do not at least partially discolour or act as a source of chemicals for migration into food. Pretreatments such as chromating, which could also colour the surface, tend not to be approved for direct food contact. However, premium foil producers wish to differentiate their product with a pleasing visual design without risking migration of foreign species into food.
The present invention addresses these problems. In one aspect the invention provides an aluminium workpiece having on a surface thereof an anodic oxide film, the thickness of the anodic oxide film being different at different predetermined locations on the surface, wherein optical interference colours are visible when the surface is viewed in white light with different colours visible at the said different predetermined locations on the surface, and wherein a metal deposit is not present within the aluminium oxide film.
A workpiece is a body of indeterminate size and shape. For example extrusions, both continuous extrusions and cut lengths, are workpieces. Formed units e.g. for architectural use or as vehicle panels are workpieces. Another example of a workpiece is a container. Sheet and plate, both in continuous form and in the form of cut pieces, are further examples of workpieces. As previously noted, the invention is mainly concerned with thin sheet of the type used for containerstock or as foil.
As discussed in more detail below, interference colour effects arise due to interference between light reflected from a metal/oxide interface at the bottom of the anodic oxide layer, and light reflected from the air/oxide interface at the top of the anodic oxide layer and/or from some semi-transparent reflective layer supported on or in the anodic oxide layer. Interference colour contrasts are generated when the thickness of the anodic oxide film is different at different predetermined locations on the surface.
The invention provides aluminium foil in the form of a continuous strip or of cut sheet at least 1 m2 in area, and having on a surface thereof an unsealed anodic oxide film 5-1000 nm thick. The anodic oxide film gives an attractive finish that also imparts wear resistance and results in a clean finish.
Aluminium sheet has in the past been continuously anodised mainly for two purposes. In one application, continuous anodising is performed at low line speeds to make thick protective anodic oxide films for architectural use. Such films comprise a barrier layer and an overlying porous layer, and the pores are sealed prior to use. The second application is performed at higher line speeds to make thin pretreatment layers to promote the adhesion of organic coatings to the sheet. Such films are typically 100 nm or less thick and are made in phosphoric acid or a similar electrolyte having substantial dissolving power for aluminium oxide, so that the resulting film is extremely rough with pillars or whiskers which provide a keying effect and promote adhesion. The anodic oxide films carried by aluminium sheet of the present invention are quite different from either of these prior art structures.
In another aspect the invention provides a method of anodising aluminium foil, which method comprises passing the foil round at least one roll immersed in anodising electrolyte and facing a series of at least three pairs of electrodes with anodising current applied between the two electrodes of each pair.
In another aspect the invention provides a process for cleaning aluminium foil, which process comprises anodising the foil in the form of continuous strip or of cut sheet at least 1 m2 in area so as to form on a surface thereof an unsealed anodic oxide film 5-1000 nm thick.
The word aluminium is herein used to cover the pure metal and alloys in which Al is a major component. The alloy may be for example in the 1000 or 3000 or 5000 or 6000 or 8000 series of the Aluminum Association Register. For example, AA 3003 and 8008 are often used for containerfoil, and AA 1100 and 1200 and 8006 are often used for foil.
Containerstock is thicker gauge foil often 80-150 xcexcm or thicker that is formed into usually open containers for food or into food trays. The term aluminium foil is generally used to denote sheet below 150 xcexcm thick. The invention is more particularly concerned with foil below 85 xcexcm thick. Domestic cooking foil is typically 8-40, and usually 10-20, xcexcm thick. Thin aluminium foil may be laminated onto a carrier such as plastic sheet for some applications such as packaging and food wrapping. The aluminium foil in this laminate is about 3-20 xcexcm usually 5-20 xcexcm thick.
Present on one or both surfaces of the sheet (but usually on one surface only) is an anodic oxide film. The film needs to be thick enough to provide fretting resistance, and should not be so thick that there is any risk of it spalling off when the foil is folded or flexed. A preferred range is 50-500 nm particularly 100-500 nm thick. Within this, films at least 125 nm thick have a major additional advantage that constructive interference colours are generated, between the metal/oxide interface on one side of the film and the oxide/air interface on the other and markedly enhance the appearance of the sheet. The pores are unsealed, i.e. they have not been formally sealed e.g. by being exposed to boiling water or steam nor to a proprietary cold sealing solution, although the pores may sometimes be found to be partly or wholly closed at their outer ends. Sealing would be impractical at the high line speeds required for economic operation.
Fretting resistance is usually an important requirement, especially in food containers. It is therefore generally preferred that such workpieces according to this invention have either one or both surfaces completely covered with an anodic oxide film. For household foil below about 40 xcexcm thickness, fretting is less likely to be a problem. Foil or sheet according to the invention may have one surface (or even both surfaces) partly or preferably wholly covered with an anodic oxide film, which film may vary in thickness over different regions of the surface as discussed below.
The anodic oxide film could in principle be of the barrier layer type, grown in an electrolyte having no dissolving power for aluminium oxide. In such systems, film thickness is proportional to anodising voltage, and inconveniently high voltages might be needed to generate films of the required thickness. So preferably the anodic oxide film is of the type formed in an electrolyte having some dissolving power for aluminium oxide and comprises a barrier layer and an overlying porous layer. It is desired that the anodic oxide film reduces, rather than increases, any tendency for food to adhere to the aluminium sheet. With this in mind, the anodic oxide film should preferably have a rather smooth outer surface, and in particular the pores should not be substantially enlarged at their outer ends. This may best be achieved by anodising the aluminium foil in an electrolyte based on sulphuric acid rather than phosphoric acid. This results in an anodic oxide film containing sulphate ion. Oxalic acid may be used as well as or instead of sulphuric acid.
Aluminium foil is frequently made by pack rolling, which results in a product having one glossy and one matt surface. While it is possible to grow an anodic oxide film on either surface, a film grown on the glossy surface or more probably the matt surface may be capable of generating brighter interference colours. A condition for strong interference colours to be visible is that the intensity of the interfering reflections of the oxide/metal and air/oxide interfaces be comparable. Thus, anodic films on relatively pure and bright Al surfaces do not normally generate interesting colours. The oxide is so transparent and the metal so reflective that the ratio of reflectance at the air interface to that of the metal interface is only about 10:90 or worse. This generates negligible xe2x80x9cfringe contrastxe2x80x9d i.e. the differences in reflectance at the wavelength of peak reflectance (constructive interference) relative to that at the adjacent minimum, i.e. the reflectance spectrum has a few small amplitude wiggles superimposed on a high and relatively flat background. There are various ways of increasing interference colours intensity in anodic oxide films on aluminium metal:
1. Decrease the metal interface reflection. This may be done by ensuring that the metal surface is rough or matt, e.g. as a result of pack rolling or etching or rolling with rough rolls.
2. Modify the anodic oxide film so that it has some intrinsic absorptivity i.e. less light gets through the film to be reflected at the metal interface. The absorptivity of the anodic oxide film may depend on the composition of the Al alloy being anodised and/or of the anodising electrolyte.
3. Provide a semi-transparent reflective layer supported by the anodic oxide film, either within the pores or more preferably on the outside surface thereof. A semi-transparent reflective layer of this kind can be created, for example, by sputtering aluminium or another metal, or by electro deposition or electroless or immersion plating to create a metal or other pigment layer a few nm thick on or in the anodic film. Techniques for doing this are described in U.S. Pat. Nos. 5,112,449 and 5,218,472. Such semi-transparent reflective layers can enhance interference colour effects and can also create dichroic effects, i.e. a surface appears coloured differently when viewed from different angles. Such semi-transparent reflective layers can if desired be provided at predetermined locations on the surface, either by selective localised application or by selective localised removal of the (metallised) layer e.g. chemically or by abrasion.
Aluminium foil may carry a silicone or waxy organic film overlying the anodic oxide film or the semi transparent reflective layer. Such a film may assist feeding the film into machinery or may resist marking for example by fingerprints. Or the surface of the foil not carrying the anodic oxide film may be laminated e.g. to paper or polymer or coated with an organic lacquer. Preferably the aluminium foil consists of a metal substrate with one surface carrying an anodic oxide film and the other surface in a natural state (i.e. with a thin naturally occurring oxide film or a thinner anodic film) and no organic or other coating.
Aluminium sheet of this invention is in the form of cut sheet at least 1 m2 or preferably at least 10 m2 in size, or of continuous sheet of indeterminate length. While it is a simple matter to anodise a laboratory sample of aluminium sheet, anodising continuous thin sheet or foil is not simple, either technically or economically. Since the areas treated are of the order of hundreds of millions of square meters per year, a very high speed process is required, e.g. at least 100 m/min and preferably 300 m/min or even more preferably 500 m/min. The film needs to be grown to a thickness of at least 65 nm to get destructive interference and preferably to 125-500 nm for the more intense, and therefore more attractive, constructive interference bands.
Of course, an anodic oxide film of uniform thickness produces an interference effect which is monochromatic. Visually striking effects can be achieved by varying the anodic oxide film thickness over different predetermined locations on an aluminium workpiece. For example, variations in film thickness of at least 10 nm e.g. some tens of nm over regions spaced some mm or cm apart e.g. up to 10 cm are found to produce striking interference colour contrasts, and this is true even if individual interference colours are faint.
While it is desired that the whole of one surface of an aluminium sheet be covered by an anodic oxide film, it is by no means necessary that the whole of that film be of thickness suitable for generating interference effects. Some regions of the surface may be colourless, e.g. by being too thin or alternatively too thick to generate interference effects. Localised variation in anodic film thickness across the width of the sheet can be achieved by careful positioning of counter electrodes in an anodising electrolyte bath through which a continuous aluminium sheet is passed. Variation in anodic film thickness along the length of the sheet can be achieved by changing the total coulombic input along the length. With suitable semi conductor control of the power supply to the anodising bath, it is possible to make quite rapid changes to the voltage and obtain closely spaced bands of colour along the length of the sheet.
Longitudinal interference colours may be generated for example, by shaping or positioning a counterelectrode so that its distance from the foil varies across the width of the foil, or so that its length (in the direction of travel of the foil) varies over the width of the foil.
This can also be achieved by masking off portions of the counter electrode in the longitudinal direction, so that the current has to travel through correspondingly more electrolyte to some parts of the foil surface than to other parts. Where the differential film growth rate is dependent on the distance travelled through the electrolyte in these arrangements the visual banding produced is strongly dependent on the throwing power of the electrolyte, and hence its composition and temperature. It is clear that geometries that allow narrow electrode gaps will therefore produce the most variation in surface finish as a greater differential resistance can be created from the electrode to the aluminium surface. However, when designing these systems it should be borne in mind that using resistance increase is less energy efficient than increasing the length of the electrode over parts of the foil width.
Variation in film thickness and therefore colour in the transverse direction can be achieved by varying the voltage (and hence current density) applied across part or all of the strip, in a manner related to the velocity of the strip. This may be done by programming a semiconductor controller such as a thyristor. Combining the longitudinal and transverse methods of control allows step changes or smoothly varying stripes of colours, such as sine waves, to be formed down the length of the strip.
Where sharp changes in colour are required, it is possible to hold a rubber mask against the surface of the strip. The mask may be pattern cut to allow images or messages. The masking can be achieved by simply interleaving the strip with a non-conductive material such as a polymer. In this manner, colour changes can be achieved that are not in the same sequence as the normal rainbow spectrum.
The invention is concerned with relatively thick oxide films, and for a continuous high speed process requires quite high current densities to keep the electrolytic cell to an economic length. Foils for domestic use are often in the 10-15 xcexcm gauge range of thickness and as such are quite resistive along their length. Power costs are a significant factor in such a process. However, to grow a film of a certain thickness requires a minimum charge density and so, for economy of operation, the voltage losses need to be kept to a minimum in order to minimise the power required. Economic cell design therefore demands that a short current path is essential.
To support such thin foils and prevent breakages a cylindrical cell geometry is preferred. In one embodiment the cylinder is also a contact roll. The centre portion of the roll needs to be conductive while the outer edges that are overlapped by the foil have to be non-conductive in order to prevent direct passage of the current from the counter electrode to the contact roll. At speeds in excess of 70 m/min arcing has been a problem in the past with contact rolls, and careful design to avoid this is necessary.
In another embodiment the cylinder is non-conductive. The foil is passed round at least one roll immersed in anodising electrolyte and facing a series of at least three pairs of electrodes with anodising currents applied between the two electrodes of each pair. In order to maintain a short current path within the foil, a small diameter squeegee roll can be used to electrically isolate counter electrodes of opposite phase. At the production speeds envisaged, the series of electrodes needs to have collectively a length, measured in the direction of travel of the aluminium foil, of 5-50 m, e.g. about 20 m. This is too long to be supplied through a single pair of electrodes. So a series of 3 to 10 pairs of electrodes is envisaged, typically about 5 pairs of electrodes. Each pair of electrodes may face a different cylinder round which the aluminium foil is led, and may be positioned in the same or different electrolyte baths.